Grain refining and increasing the Ni content are the principal metallurgical methods for improving the brittle crack propagation arrest characteristics of a steel plate. Increasing the Ni content is a method for improving the brittle crack propagation arrest characteristics without relying on the microstructure, but the method naturally brings about an increase in the cost. Accordingly, grain refining by devising a production process is preferred. It is concluded from the brittle crack propagation-arrest behaviors of steel plates as a whole that what actually contributes greatly to the improvement of the brittle crack propagation arrest characteristics is a plastic deformation region termed a shear rip formed in the surface layer portions of the steel plate during brittle crack propagation, and that when the shear rip is formed, the ability of the steel plate for absorbing the propagation energy that the brittle crack has is increased and the brittle crack propagation arrest characteristics are greatly improved. The formation of the shear rip is achieved by grain refining.
Accordingly, various attempts have heretofore been made to improve the brittle fracture propagation arrest characteristics by grain refining. In general, grain refining is effected by increasing the degree of controlled. rolling in hot rolling, or adding Nb to further facilitate controlled rolling. However, increasing the degree of controlled rolling brings about lowered productivity, and adding Nb is likely to result in the deterioration of toughness in a weld zone. Moreover, significant grain refining cannot be expected by these methods, and the effect of improving the brittle crack propagation arrest characteristics thus obtained is small. Recently, for example, Japanese Patent Publication Kokai No. 61-235534 proposes a process for producing a steel plate exhibiting a Kca value, which represents a brittle crack propagation arrest characteristics at -20.degree. C. by ESSO test, of about 460 to 960 kgf.multidot.mm.sup.-3/2, by cooling the steel slab from the surface to a distance corresponding to at least 1/8 of the slab thickness in the central part at temperatures up to Ar.sub.3 transformation temperature, starting rolling while the temperature difference is maintained in the thickness direction of the steel slab, and recuperating the steel to temperatures of at least the Ac.sub.3 transformation temperature in the entire region of the steel slab thickness during rolling or after rolling.
Accordingly, steel products are required to have higher brittle crack propagation arrest characteristics as the structures now tend to be used in harsher environments. The characteristics of a steel plate attained by the process mentioned above, therefore, may not always be satisfactory. In the process of Japanese Patent Publication Kokai No. 61-235534, the entire region of the steel slab is simply recuperated to temperatures of at least the AC.sub.3 transformation temperature, and the .alpha.-grain size finally obtained by .gamma.-.alpha. transformation is about 5 .mu.m at the least. Accordingly, a new technique is required to further improving the brittle crack propagation arrest characteristics.
There has been proposed, very recently, a process wherein the surface layer portions of a steel are cooled and then subjected to significant grain refining by rolling during recuperation to improve the brittle crack propagation arrest characteristics, as disclosed in Japanese Patent Publication Kokai No. 4-141517. According to the process, the surface layer portions are made to have ultrafine grains on the average, and a shear rip is formed therein, whereby excellent brittle crack propagation arrest characteristics are achieved even at -50.degree. C. However, since ultrafine grains are formed principally by work recrystallization of ferrite during recuperation, there has been found a problem in that a structure and a steel material with nonuniformity are likely to be formed due to a delicate variation of the heat cycle. Although the surface layer portions of the steel plate have come to have a grain size of 3 .mu.m level, which level is as fine as about 1/3 to 1/10 of the grain size level of conventional steel plates, complete prevention of brittle fracture cannot be attained in a certain temperature range where the steel plate is used. A very good toughening technique is newly required in addition to mere grain refining.